This invention relates to transformers of the type particularly useful in supplying power to a discharge lamp from a high frequency push-pull inverter power supply. In particular, this invention relates to transformer structures providing integral ballasting reactance and to a winding method which yields coupling close to that provided by bifilar winding but is more readily manufactured and which reduces interturn voltage differences.
The so-called push-pull square-wave inverter power supply for providing alternating current energy to an electric discharge lamp is an energy efficient circuit but one that requires transformer coupling to the discharge tube. In particular, the primary of the transformer comprises two portions which must be intimately coupled with one another. The two portions of the primary winding are typically serially connected forming a standard center-tapped primary winding, each winding being driven in an alternating fashion by the power output transistors in the push-pull inverter. In prior art transformers operating in conjunction with push-pull inverters, the portions of the primary winding must be bifilarly wound. However, the bifilar winding process is difficult, time-consuming, and expensive, especially as compared with more standard winding processes.
Moreover, in lamp discharge circuits, it is also necessary to provide a ballasting reactance to compensate for the fact that an initial high voltage is needed to initiate the discharge but a much smaller voltage is needed to sustain the discharge after ignition. This ballasting reactance is provided by a variety of prior art structures. For example, if a lumped (that is, multi-layered) primary and a lumped secondary winding are employed encircling a transformer core structure, then there is typically a certain portion of magnetic flux returned through the atmosphere rather than through the core, and the necessary ballasting reactance is provided by this leakage reactance due to imperfect primary and secondary coupling. The amount of ballasting leakage reactance is controlled by selectively adjusting the relative positions of two lumped primary and secondary windings. However, in prior art lumped winding structures, the primary center-tapped winding must be wound in a bifilar fashion to achieve the requisite degree of close coupling between the two primary portions.
Another method for providing the necessary ballasting reactance is by providing a second and separate ballasting inductor; but this configuration also needs bifilarly wound primary windings. However, this method consumes a greater amount of material and in addition takes up space especially in those discharge lamp applications in which the lamp, the ballast, and power supply are provided in an integrated structure.
For the case that a separate ballasting inductance is not provided, the requirement of a relatively large ballasting reactance implies that a large number of turns are needed with the primary circuit being wound as a lumped coil. This requirement for a large number of turns and the lumped coil also renders unpractical certain other primary winding methods which would otherwise be usable. In particular, one method of achieving close coupling between the two portions of the primary winding is to have one winding overlay the other rather than have the interleaved, but preferred, bifilar pattern. However, if the number of turns is required to be large in order to provide a large ballasting reactance, then an overlaid winding pattern becomes costly because of the number of layers required.
Thus, prior art transformers, particularly those used with high frequency inverters providing power to discharge lamps, are unable to supply sufficient ballasting reactance while at the same time permitting simple primary winding methods. In addition, prior art transformers in discharge lamp circuits suffer from an undesirable amount of energy loss due to leakage magnetic fields inducing currents in surrounding metal structures such as the ballast case.